In general, wide-angle zoom lenses of 2× or higher in variable power in the state of the arts are of optical architecture primarily having three groups of lens pieces arranged to assume negative, negative, and positive refractivities.
For instance, there has been disclosed a zoom lens of three groups of lens pieces, namely, the 1st to 3rd lens groups, where the 1st lens group assumes negative refractivity, as a whole, and includes the foremost or the first lens piece of negative refractivity closest to objects, the 2nd lens group includes two serial lens pieces succeeding to the first, namely, the second lens piece of positive refractivity and the third lens piece of negative refractivity closer to the image plane, and the 3rd lens group assumes positive refractivity, as a whole, and includes four serial lens pieces, namely, the fourth lens piece of positive refractivity, the fifth lens piece of positive refractivity similar to the fourth, the sixth lens piece of negative refractivity joined to the fifth in a composite lens unit, and the seventh lens piece of the reduced negative refractivity or the reduced positive refractivity. In such a zoom lens, the power is varied by displacing the 1st, 2nd and 3rd lens groups (see Patent Document 1 listed below).
Although an amount of the displacement of those lens groups must be greater and the number of the lens groups must be increased in order to raise the zooming power, another prior art wide-angle zoom lens already disclosed is approximately 75 degrees in field angle and attains the variable power higher than 2×, having its optics relatively reduced in total extension without increase in the space confining the lens groups displaced for varying the power, where there are three groups of lens pieces of negative, negative, and positive refractivities arranged in this order on the closer to objects first basis (see Patent Document 2 listed below).
Further another prior art zoom lens, offering a wider field of view at the wide-angle end, has been disclosed which is comprised of three groups of lens pieces, namely, the 1st lens group of negative refractivity, the 2nd lens group of negative refractivity, and the 3rd lens group of positive refractivity arranged in this order on the closer to objects first basis where the 2nd and 3rd lens groups are displaced along the optical axis for the zooming (see Patent Document 3). This zoom lens makes a wide-angle photo as much as 112 degrees in field angle available, and its Leica 35-mm version can implement a focal length of 14 mm for that field angle.
Still another prior art wide-angle zoom lens, characterized by the field of view as wide as 80 degrees at the wide-angle end, has been disclosed, which has three lens groups respectively assuming negative, negative, and positive refractivities and arranged in this order on the closer to objects first basis (see Patent Document 4 listed below).
Patent Document 1
Japanese Patent Publication of Unexamined Application 2006-301393
Patent Document 2
Japanese Patent Publication of Unexamined Application H2-259614
Patent Document 3
Japanese Patent Publication of Unexamined Application H4-275515
Patent Document 4
Japanese Patent Publication of Unexamined Application SHO53-103753
The zoom lens as disclosed in the cited Patent Document 1, which includes the lens groups of negative, negative, and positive powers and has the variable power higher than 2×, is advantageous in that the entire optics is reduced in extension, but its field angle of view is at most 75 degrees and cannot attain any further greater field angle as desired in the industry.
The zoom lens as disclosed in the cited Patent Document 2, which includes the lens groups of negative, negative, and positive refractivities, is at most 75 degrees in its widest field angle of view, and this is far behind the further greater field angle as currently desired in the art.
The zoom lens as disclosed in the cited Patent Document 3 has the widest field angle of view as much as 112 degrees that can be implemented by the 14-mm focal length in the Leica 35-mm version, but regretfully its varied power is at most 1.3× or so.
The zoom lens as disclosed in the cited Patent Document 4 has the negative-negative-positive multi lens group configuration and permits photographing of the field of view as wide as 80 degrees at the wide-angle end, but its highest varied power is still not more than 1.6×.
The present invention is made to overcome the aforementioned disadvantages in the prior art wide-angle zoom lens, and accordingly, it is an object of the present invention to provide a wide-angle zoom lens that is capable of covering the field of view ranging from 80 degrees to 115 degrees in field angle with a zooming power of 2× or higher, and is capable of appropriately compensating for various types of aberration such as spherical aberration, aberration of distortion, astigmatism and the like.
It is another object of the present invention to provide a wide-angle zoom lens that has its optics reduced in total extension without increase in the space confining lens groups displaced for varying the power although an amount of the displacement of the lens groups must be greater and the number of the lens groups must be increased in order to raise the zooming power.
It is still another object of the present invention to provide a wide-angle zoom lens that is satisfactorily downsized and reduced in manufacturing cost as desired in the market, which is attained by getting the outermost primary beam incident on the foremost lens surface at a point as close to the optical axis as possible so as to reduce radial dimensions of the optics, especially, an effective diameter of the lens piece closest to objects.
It is further another object of the present invention to provide a wide-angle zoom lens that displaces the trailing lens set (1B) closer to the image plane in the leading or 1st group of lens pieces for the transitive focusing from objects at infinity point to those at near point rather than displacing the 1st lens group in its entirety, so as to reduce an effective diameter of the 1st lens group, and that facilitates actuating the lens groups relatively reduced in dimensions and weight, during the focusing.